Currently, most existing active, double clad optical fibres utilise a low refractive index polymer cladding for coating the optical fibre. These existing double clad fibres have a core and an inner cladding, surrounding the core, for propagating radiation from a pump laser coupled in to the optical fibre, and an outer cladding, surrounding the inner cladding, for confining the pump radiation to the core and the inner cladding. The polymer cladding used in these double clad fibres is selected for its optical properties to allow a wide acceptance angle of incoming pump light; however, this polymer material generally degrades when exposed to high temperatures, such as those greater than 80° C., and has very poor thermal conductivity, such as around 0.18 W/m/K. Accordingly, there is a limitation on the level of pump power that can be coupled into such a double clad optical fibre without providing suitable cooling systems for the laser. For example, in some higher power applications, such as high power fibre amplifiers and lasers, that employ the above existing active, double clad optical fibre, cooling systems including thermo-electric coolers, water circulator and large thermal masses are used to cool the laser so as to not damage the active optical fibre; in particular, the thermally sensitive polymer cladding.
An example of a prior art active, double clad optical fibre is shown with respect to FIG. 1. As described, the core and the inner cladding of the prior art active, double clad optical fibre propagate the pump radiation therethrough, and the outer cladding confines the pump radiation to the core and the inner cladding. It will be appreciated by those persons skilled in the art that an active optical fibre includes a core that guides and enables light amplification by stimulated emission of radiation for a single mode or a multi-mode signal. The outer cladding confines the pump radiation by having a smaller refractive index than the inner cladding. FIG. 1 shows a graphical representation 10 of the relative refractive indexes of the components of the exemplary double clad optical fibre. Specifically, FIG. 1 shows a line 11 indicative of the respective refractive indexes of a central core 12, inner cladding 14, and an outer cladding 16 of the optical fibre, relative to their position in the fibre. Here, the outer cladding 16 consists of a polymer used for its optical properties to enable the guidance of pump radiation as well as to provide a physically protective coating to the optical fibre.
It can be seen from FIG. 1 that the core 12 is formed from a material with a first refractive index n1, the inner cladding 14 is formed from a material with a second refractive index n2 that is smaller than the first refractive index n1, and the outer cladding 16 is formed from a polymer material with a third refractive index n3 that is smaller again than the second refractive index n2.
It will be appreciated by those persons skilled in the art that numerical aperture or NA of an optical fibre is given by the equation: NA=√{square root over (n12−n22)} where n1 is a first refractive index and n2 is a second refractive index. It will also be appreciated that numerical aperture can be related to the acceptance angle of the optical fibre by the equation: NA=n*sin(θ) where n is the refractive index of the medium from which light is being launched. Typically, the medium is air with a refractive index equal to 1.
Turning back to FIG. 1, the acceptance aperture for receiving pump radiation from a laser coupled to the optical fibre is defined by an index difference between the second refractive index n2 of the inner cladding 14 and the third refractive index n3 of the outer cladding 16. As described, the index difference between the second refractive index n2 and the third refractive index n3 shown in FIG. 1 is relatively large and thus the acceptance aperture is also relatively large. For example, the inner cladding is pure silica with a refractive index of 1.45 and the outer cladding is a polymer coating with a refractive index of 1.373.
It will also be appreciated by those persons skilled in the art that for any wavelength dispersive medium the refractive index of this medium is dependent on the wavelength of incident light. Herein, any reference to refractive index relates to the operation at a wavelength of light of 1.064 micrometres. Nonetheless, it will also be appreciated that the reference wavelength is used for clarity purpose and does not limit the use of this wavelength within the invention. Indeed, it will be appreciated that the wavelength of operation of this invention can cover the entire spectrum of which an optical fibre is transparent.
In the prior art example shown in FIG. 1, the acceptance aperture of the optical fibre is determined, by the index difference between the second refractive index n2 of the inner cladding 14 and the third refractive index n3 of the outer cladding 16, to be 0.46 NA. This relatively large acceptance aperture of 0.46 NA enables a laser with a relatively low brightness to be coupled to the optical fibre. The polymer cladding used, however, generally degrades when exposed to high temperatures and has very poor thermal conductivity.